Location determination system having mesh infrastructure to reduce power consumption

ABSTRACT

The present disclosure relates to a location determination system that includes acoustic transmitting devices (104), location tags (112), and a wireless mesh network (106), where the wireless mesh network uses battery-powered devices. A location tag receives acoustic signals (e.g., ultrasound signals) from an acoustic transmitting device. Clocks from members of the wireless mesh network are synchronized by observation of clock pairings, each clock pair formed by respective clocks in a transmitting device that transmits a message and a receiving device that receives the message. By analyzing the observed clock pairings, a best fit between the clock pairings may be determined. After selecting a reference clock, an acoustic transmission schedule may be propagated to the respective acoustic transmitting device.

FIELD

The present disclosure relates generally to real-time location systemsand more particularly to determining the location of an object or personwithin a real-time location system.

BACKGROUND

Modern businesses and organizations face a common challenge in trackingthe location of important resources in a building or campus environment.Such resources include key personnel, critical pieces of equipment,vital records, or other useful devices. These resources often relocatethroughout the day according to organizational needs, and locating theseimportant resources can prove difficult and time consuming. In order toavoid the inherent productivity loss in devoting time and energy towardsmanually locating these resources, it is desirable to develop anapproach that tracks, catalogues, and reports the location of theseimportant resources in real-time.

SUMMARY

In an embodiment of the present disclosure, a wireless mesh network isdescribed that includes a plurality of location tags and transmittingdevices capable of transmitting and receiving a plurality of timepairings, wherein a time pairing is the pairing between a first localclock of a first device and a second local clock of a second device. Theplurality of time pairings in the wireless mesh network statisticallydetermines a relative clock drift from a synthesized reference clock foreach location tag and acoustic transmitting device in the wireless meshnetwork. The relative clock drift for each device is compared to thesynthesized reference clock and used realize time coordinated eventsbetween devices.

In a further embodiment of the present disclosure, acomputer-implemented system is disclosed that includes a plurality oflocation tags and transmitting devices configured to form a wirelessnetwork that is configured to wirelessly communicate a plurality of timepairings, wherein a time pairing is the combined storage of a localclock of a first device and a second clock of a second device. Thecomputer-implemented system also includes an acoustic transmittingdevice configured to receive the plurality of time pairings and transmitthe plurality of time pairings to a central server. Thecomputer-implemented system also includes the central server configuredto receive the plurality of time pairings and synthesize a referenceclock to which each of the device clocks can be compared.

In a further embodiment of the present disclosure, acomputer-implemented method for utilizing a wireless mesh network isdisclosed that includes storing by a first device a first local clocktime. The computer-implemented method further includes receiving by thefirst device a second local clock time from a second device in thewireless mesh network. The computer-implemented method further includestransmitting a time pairing between the first local clock time and thesecond local clock time wherein the time difference is used tostatistically determine a relative clock drift from a reference clockfor each device and the relative clock drift is used to realize timecoordinated events among the plurality of devices and transmittingdevices.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate embodiments of the present disclosureand, together with the description, further explain the principles ofthe disclosure and enable a person skilled in the pertinent arts to makeand use the embodiments.

FIG. 1 illustrates a perspective representation of a real-time locationsystem, according to example embodiments of the present disclosure.

FIG. 2 illustrates a block diagram of an exemplary mesh infrastructuresystem as used in a real-time location system, according to exampleembodiments of the present disclosure.

FIG. 3 illustrates a method of generating an acoustic transmissionschedule based on an analysis of observed clock pairings to providesynchronized acoustic transmissions for a real-time location system,according to example embodiments of the present disclosure.

FIG. 4 illustrates an example computing system according to exampleaspects of the present disclosure.

The present disclosure will be described with reference to theaccompanying drawings. In the drawings, like reference numbers indicateidentical or functionally similar elements. Additionally, the left-mostdigit of a reference number identifies the drawing in which thereference number first appears.

DETAILED DESCRIPTION

The following Detailed Description refers to accompanying drawings toillustrate exemplary embodiments consistent with the disclosure.References in the Detailed Description to “one exemplary embodiment,”“an exemplary embodiment,” “an example exemplary embodiment,” etc.,indicate that the exemplary embodiment described may include aparticular feature, structure, or characteristic, but every exemplaryembodiment does not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases do not necessarilyrefer to the same exemplary embodiment. Further, when the disclosuredescribes a particular feature, structure, or characteristic inconnection with an exemplary embodiment, those skilled in the relevantarts will know how to affect such feature, structure, or characteristicin connection with other exemplary embodiments, whether or notexplicitly described.

The exemplary embodiments described herein provide illustrative examplesand are not limiting. Other exemplary embodiments are possible, andmodifications may be made to the exemplary embodiments within the spiritand scope of the disclosure. Therefore, the Detailed Description doesnot limit the disclosure. Rather, only the below claims and theirequivalents define the scope of the disclosure.

Hardware (e.g., circuits), firmware, software, or any combinationthereof may be used to achieve the embodiments. Embodiments may also beimplemented as instructions stored on a machine-readable medium and readand executed by one or more processors. A machine-readable mediumincludes any mechanism for storing or transmitting information in a formreadable by a machine (e.g., a computing device). For example, in someembodiments a machine-readable medium includes read-only memory (ROM);random-access memory (RAM); magnetic disk storage media; optical storagemedia; flash memory devices; electrical, optical, acoustical or otherforms of propagated signals (e.g., carrier waves, infrared signals,digital signals, etc.), and others. Further, firmware, software,routines, instructions may be described herein as performing certainactions. However, it should be appreciated that such descriptions aremerely for convenience and that the actions result from computingdevices, processors, controllers, or other devices executing thefirmware, software, routines, and/or instructions.

Any reference to the term “module” shall be understood to include atleast one of software, firmware, and hardware (such as one or morecircuit, microchip, or device, or any combination thereof), and anycombination thereof. In addition, those skilled in relevant arts willunderstand that each module may include one, or more than one, componentwithin an actual device, and each component that forms a part of thedescribed module may function either cooperatively or independently ofany other component forming a part of the module. Conversely, multiplemodules described herein may represent a single component within anactual device. Further, components within a module may be in a singledevice or distributed among multiple devices in a wired or wirelessmanner.

The following Detailed Description of the exemplary embodiments willfully reveal the general nature of the disclosure so that others can, byapplying knowledge of those skilled in relevant arts, readily modifyand/or customize for various applications such exemplary embodiments,without undue experimentation and without departing from the spirit andscope of the disclosure. Therefore, such modifications fall within themeaning and plurality of equivalents of the exemplary embodiments basedupon the teaching and guidance presented herein. Here, the phraseologyor terminology serves the purpose of description, not limitation, suchthat the terminology or phraseology of the present specification shouldbe interpreted by those skilled in relevant arts in light of theteachings herein.

The present disclosure provides a real-time location system that tracksaccurate location information of objects and persons. Real-time locationsystems operate with different levels of accuracy depending on availablesystem infrastructure. In some embodiments, room-level accuracy oflocation information is sufficient. In some embodiments, a systemprovides three-dimensional location information about persons orequipment in real time. A real-time location system can include anetwork of acoustic transmitting devices attached to interior surfacesin an environment and location tags attached to moveable objects orpeople. The location tags receive signals from the acoustic transmittingdevices to determine descriptive location information orthree-dimensional locational coordinates within the environment.Acoustics, e.g., ultrasound, is well-suited for this purpose because ittravels slower than radio waves and generally goes unnoticed by humans.Acoustic waves also attenuate more rapidly and are less likely topenetrate walls, which minimizes signal interference between rooms. Ifthe location of the acoustic transmitting device is known, then thelocation of the location tag is in the near vicinity of the location ofthe particular acoustic transmitting device whose signals have beenreceived. For example, if a particular acoustic transmitting device hasbeen installed in a closed room and a location tag receives the acousticsignal from the particular acoustic transmitting device, then thelocation tag is located within the closed room. Acoustic signals arealso easier to process when measuring a relatively short distance, asexists between the acoustic transmitting devices and the location tagsdescribed herein.

The systems and methods of the present disclosure can be used in anumber of applications including, for example, location tracking, workflow, mobile equipment tracking, safety and compliance, mobile equipmentmanagement, staff location determination, or other suitableapplications. One example field of use is within the health careindustry. In an embodiment, a hospital implements a real-time locationtracking system of the present disclosure to provide patient tracking,patient flow, asset management, environmental monitoring, etc.

Implementation of a real-time location system requires installation of anumber of acoustic transmitting devices throughout the environment forwhich location determination is desired. These acoustic transmittingdevices provide the necessary ID messaging that is received by thelocation tags, which are then used to provide location determination ofthe location tags. In addition, these acoustic transmitting devices needto be synchronized in time, and to be configured (e.g., periodicity ofultrasound transmissions, firmware upgrades, etc.). In an exemplaryembodiment, the synchronizations in time and the configurationmanagement are effected by a central server. For example, a centralserver will maintain a reference clock to which the internal clocks ofthe acoustic transmitting devices are synchronized. Similarly, updatesto the configuration of these acoustic transmitting devices areroutinely required, and are typically provided via the central server.The central server is merely exemplary, and any alternative centralizedentity may be used to support the centralized functions.

To support the synchronization and the configuration management, thenecessary communication paths to support the communication between theacoustic transmitting devices and the central server may be accomplishedusing a number of intermediate nodes, e.g., gateways. Using RF or othercommunication links, various communication messages between the acoustictransmitting devices and location tags are forwarded to the gateway thatprovides a footprint that covers the respective acoustic transmittingdevices and location tags. Those communication messages are in turnforwarded to the central server. A typical arrangement for thecommunication pathways between the gateways and the central server is astar topology. In a star topology, each gateway is connected to thecentral server via a point-to-point connection. Such a topology offersvarious benefits, such as a simplicity in adding (or subtracting)additional gateways. In a typical star topology implementation, thepoint-to-point connections may be Ethernet connections and theindividual gateways may be powered by power supplies that are, in turn,connected to the mains power supply. Power may also be supplied using apower-over-Ethernet approach. However, both the Ethernet connections andthe power supply arrangements require extensive cabling to each andevery gateway, and therefore this presents a disadvantage to the use ofa star topology connection approach.

However, the required cabling presents unique challenges. Cost is afactor when installing the required gateways and other infrastructurewhere a star-network topology is utilized. In particular, the cost ofcabling the gateways is substantial. For example, in an exemplaryreal-time location system, installation of 300 gateways could cost inthe vicinity of two to three thousand dollars to provide each requiredpower drop, and thereby upwards of one million dollars to provide thenecessary installation. Such an installation may be good only for a fewyears, at which time retrofits of floor space demand a re-deployment ofthe gateways and a subsequent further expenditure of significant monies.In addition, power consumption among the gateway devices can also becomeexcessive.

Therefore, a need exists for real-time location systems to provideaccurate location information of location tags in an environment withreduced infrastructure requirements, e.g., a need for an approach thatreduces the cabling and other costs associated with the infrastructurenetwork of supporting connectivity of the acoustic transmitting devicesand the location tags is desirable. The approach described hereinreduces power consumption and dramatically reduces installation costsmaking the real-time location system more easily deployable, scalable,and efficient.

Embodiments of the approach described herein rely on battery-poweredacoustic transmitting devices, and employ a wireless approach toconnectivity from the various acoustic transmitting devices to thecentral server. Wireless connectivity is limited by the physicalfootprint over which wireless transmission may be reliably received.Given the size of exemplary environments (e.g., hospital) for whichreal-time location is desired, point-to-point wireless connectivity in astar topology is not feasible. The alternative approach may use a meshnetwork connectivity approach, where each connection between one endnode (i.e., acoustic transmitting device) and a neighboring node may beaccomplished by a wireless connection. In summary, having a system wherethe battery-powered infrastructure self-organizes into a mesh network ishighly advantageous in reducing power consumption and the cost ofinstallation. Use of a self-organizing mesh network for communicationbetween the location tags and acoustic transmission devices and thecentral server either eliminates the need for the intermediate gateways,or substantially reduces the need for such intermediate gateways. Thecommunication pathways in the mesh network are used to time-synchronizea number of transmission windows (e.g., the time windows during whichacoustic transmissions are made by the acoustic transmission devices tolocation tags, the time windows during which RF transmissions betweenneighboring acoustic transmitting devices) across the real-time locationsystem. Time synchronization among all members of the wireless meshnetwork to a reference clock (that may be located in a central server)allows the location tags and the acoustic transmission devices toutilize power only for a time sub-interval during which a transmissionactually occurs. In addition, it allows for time of flight estimation(TOF) of the acoustic signals exchanged between transmitting deices andlocation tags. TOF data can then be used to estimate position data forthe location tags and transmitter devices.

Mesh networks work best in situations where the nodes (e.g., gateways)have continual access to power, and therefore will unfailingly passmessages from end nodes back to a server. In contrast, mesh networks arenot reliable for backhaul purposes to dependably convey messages fromend nodes back to a server via intermediate nodes that are batterypowered. In particular, battery powered nodes that enter a sleep modeare difficult intermediate nodes as their sleep cycle may not becoordinated to provide a reliable means for backhaul of criticalmessages to the server. Battery powered nodes are particularlyvulnerable when the messages contain the information that is used toprovide a synchronized sleep schedule by which the battery powered nodeswill be awake to reliably forward the messages from which thesynchronization schedule is derived. Thus, while the benefits ofbattery-powered nodes are clear in that the cost of a buildout of suchnodes is substantially less than the equivalent constantly-powerednodes, the design of timing messages that use such battery-powered nodesin the backhaul link is not straightforward. It is with these challengesin mind that embodiments of the approach described herein weredeveloped.

Real-Time Location Systems

Real-time location systems have been developed using various wirelessprotocols with perhaps the best known system being the globalpositioning system (GPS). While such location systems provide horizontallocation accuracies on the order of approximately 8 meters, thesesystems do not solve all location-tracking scenarios. For example, manyscenarios require location accuracies of less than 0.3 meters. Otherscenarios require the ability to distinguish between floors in ahigh-rise building. Still other scenarios necessitate contextuallocation information, such as room-based information in an officebuilding.

Embodiments of the present disclosure provide solutions to theseaugmented location requirements. Acoustic signals may be used todetermine the location of location tags, attached to persons or objects,by transmitting acoustic signals from acoustic transmitting devices,which may be fixed to the walls or ceilings of a building. In anexemplary embodiment, the location of the location tags may bedetermined in three dimensions. In a further exemplary embodiment, theacoustic signals include identifiers (including encoded identifiers)assigned to their respective acoustic transmitting device. The locationtags detect or decode the identifying information in the acousticsignals. If the location of the acoustic transmitting device is known,then the location of the location tag is in the vicinity of the locationof the particular acoustic transmitting device whose signals have beenreceived. For example, if a particular acoustic transmitting device hasbeen installed in a closed room and a location tag receives the acousticsignal from the particular acoustic transmitting device, then thelocation tag is located within the closed room. Therefore, if each ofthe acoustic transmitting devices emits a unique identifying signal andthe location of each acoustic transmitting device is known, then thelocation of a location tag may be determined upon its receipt of anacoustic signal from a particular acoustic transmitting device and itsassociated identity.

Similarly, if a location tag receives acoustic signals from each of twoseparately identifiable acoustic transmitting devices, then the locationtag is located in the vicinity of both of the two separatelyidentifiable acoustic transmitting devices. For example, were the twoseparately identifiable acoustic transmitting devices placed at the twoopposite ends of a corridor, then a location tag situated in thecorridor would likely receive signals from the two separatelyidentifiable acoustic transmitting devices, and the location of thelocation tag would be established. In other embodiments, more preciselocations of a location tag may be established. For example, arrivaltimes of acoustic signals at a location tag can be used to find thelocation of the location tag in the environment, using standardgeometric calculations.

In an embodiment, the acoustic signals may also include data associatedwith the environment near the acoustic transmitting device such as oneor more rooms, spaces, structures, buildings, regions, etc. in which theacoustic transmitting device resides. More particularly, suchenvironmental data can include specific details associated with theenvironment. For instance, the environmental data can indicate therespective room, building, campus, area, etc. where the acoustictransmitting device is located. The environmental data can furtherinclude data specifying an organization, configuration, or hierarchy ofthe environment in which the acoustic transmitting device is located.For instance, such environmental data can include data specifying arelationship between a particular room and a particular building (e.g.,a location of the room within the building).

The environmental data can further include dimensional data associatedwith the environment. For instance, dimensional data can include thedimensions of one or more reflective surfaces (e.g. walls, ceilings,floors, objects, furniture, etc.) within a room in which a transmittingdevice is located. The dimensional data can further include dataindicative of the normal direction of the reflective surfaces. Theenvironmental data can further include data indicative of the acousticattenuation of such reflective surfaces. The environmental data canfurther include data indicative of the relative locations of theacoustic transmitting devices within a particular room, building, area,etc. More particularly, such environmental data can include anidentifier of a surface (e.g. wall, floor, ceiling, etc. of a room) onwhich a transmitting device is located and/or data indicative of alocation and/or orientation of the transmitting device with respect tothe surface. The environmental data can further include atmospheric dataindicative of the speed of sound, temperature, pressure, humidity, etc.within the environment. In certain embodiments, since the environmentaldata changes over time, the environmental data updates frequently toreflect current environmental conditions.

The acoustic transmitting devices of the real-time location system canbe configured to periodically transmit acoustic signals (or othersuitable signals, such as radio frequency signals) to be received bylocation tags located within broadcast range of the transmittingdevices. In some implementations, the acoustic signals can be ultrasonicsignals having a frequency of about 20 kHz or greater. In a particularembodiment of the present disclosure, the acoustic signals can beultrasonic signals having a frequency of about 20 kHz. In anotherparticular embodiment of the present disclosure, the acoustic signalscan be ultrasonic signals having a frequency of about 40 kHz. As usedherein, the term “about,” when used in reference to a numerical value,refers to within 30% of that value.

In this manner, a location tag within the broadcast range of theacoustic transmitting devices picks up acoustic signals. The acousticsignals can be signals propagating directly from the acoustictransmitting devices to the location tags (referred to herein as “directsignals”) and/or signals reflected by one or more reflective surfaces(referred to herein as “reflected signals”). The reflective surfaces canact as sonic mirrors capable of reflecting acoustic signals (with someattenuation) and can include walls, ceilings, floors, furniture,objects, etc. located within the environment. The precise location of alocation tag can be determined based at least in part on the acousticsignals received from the acoustic transmitting devices. In someimplementations, contextual or descriptive location information may beprovided, e.g., room number or floor number in an office building.

Maintaining time synchronization among the acoustic transmitting devicesand the location tags is essential in the above-described locationdetermination system in order to minimize power consumption and reduceinfrastructure requirements. In addition, it allows for time of flightestimation (TOF) of the acoustic signals exchanged between transmittingdeices and location tags. TOF data can then be used to estimate positiondata for the location tags and transmitter devices. In an embodiment,location tags maintain time synchronization with the acoustictransmitting device that they are associated with by listening for theacoustic signals sent by the acoustic transmitting devices within acoordinated subinterval of time. However, in an alternate embodiment,the location tag may obtain similar information by transmitting a proberequest to a central server that replies with information on identities,configurations, and clock values.

In an embodiment, arranging the location tags and acoustic transmittingdevices into a self-organizing, self-determining wireless mesh networkprovides further advantages. Where a location tag communicates with awireless access unit, additional infrastructure must be installed to addwireless access units and provide sufficient power to them. Suchinfrastructure could include gateways, power drops, hubs, wiring,Bluetooth beacons, antennas, etc. Unit-to-unit communication allows theneeded information to propagate from the location tags and acoustictransmitting devices without the need for any additional infrastructureto be installed in the environment. In this manner, a timesynchronization system can function independently of the existinggateways either by synchronizing time locally or at a central observer.Embodiments provide synchronization information by returning calculatedtime offsets and drifts (with respect to a reference clock) to themembers through the mesh network itself.

The wireless mesh network is self-organizing because location tagsreceive and transmit packets containing information about local clocktimes without any centralized coordination. The members of the wirelessmesh network neighbors within transmission range and transmit timestamppairs and local time stamps to the neighbors. The timestamp pairsconsist of the local clock time and the local clock time of theneighbor. A local time stamp is the time on the mesh network memberprior to comparison or differentiation with respect to any other memberof the mesh network. Eventually, these timestamp pairs reach a centralserver, which calculates the clock offset and drift for all of thelocation tags and transmitter devices using known statistical methods,which will be understood by one skilled in the art.

Device Clock Tracking

In an exemplary wireless mesh network, the wireless nodes comprise oneor more location transmitters. The exemplary mesh network is connectedto a server that provides various centralized functions for thereal-time location system. The nodes in the mesh network are positionedthroughout an environment so that the wireless coverage footprint ofeach node in the network overlaps with the wireless coverage footprintof at least one other node. The overlapping wireless footprints ensurethat there is network connectivity from any one network node to anyother network node in the mesh network, and more particularly betweenany one network node back to the server, which is coupled to thenetwork. In certain embodiments, the wireless mesh network connects withthe server via a gateway. In other embodiments, the wireless meshnetwork may directly connect to the server. In these embodiments, theserver has an internal wireless transceiver that is a node within thewireless mesh network. In further embodiments, an environment (e.g., abuilding) may be divided into two or more separate clusters. Byseparate, it is meant that the acoustic footprint of one cluster doesnot overlap with the acoustic footprint of another cluster. In theseembodiments, all units in one cluster would be synchronized with eachother, while units from this one cluster are not synchronized with unitsfrom another cluster. A single server may perform the analysis tosupport the synchronization of the units with each cluster.

Each node in an exemplary wireless mesh network may contain a radiomodule for provision of the wireless transmission and receptionfunctions. The radio module includes a timer that is locked to a crystaloscillator to provide a stable time reference. This timer may bereferred to as a device clock. In an exemplary embodiment, the crystaloscillator may have a frequency of 32 kHz, with 10 ppm (parts permillion) accuracy. The 32 kHz frequency is merely exemplary, and notlimiting. With a frequency of 32 kHz, each clock tick will beapproximately 0.03 milliseconds. Using a 24-bit word for the output ofthe timer, and the nominal frequency of 32 kHz, the timer word valuewill wrap around (i.e., repeat) approximately every 10 minutes. Timerresets result in a word value being reset to an initial starting point,e.g., 0. With the exemplary 10 ppm accuracy, the timer will accumulativean error of approximately 1 millisecond over a duration of 100 seconds.Since the acoustic (e.g., ultrasonic) transmission schedule of areal-time location system requires synchronization across the deviceclocks (device timers) in the real-time location system, suchsynchronization requires that an on-going compensation for each devicetimer be provided to compensate for device clock drift and clock offset(e.g., device resets).

In an embodiment, compensation for device clock drift and clock offsetmay be derived from timestamp pairings. A pairing results when a messageis broadcast wirelessly from one device (D1) and received by a seconddevice (D2). When device D1 broadcasts a message, such a message mayinclude a timestamp from the clock of the originating device D1. Uponreceipt of the message, a timestamp from the clock of receiving deviceD2 may be added to the message. Consequently, with each receivedmessage, a pair of clock timestamps is observed, one timestamp from thebroadcasting device and one timestamp from the receiving device.Observation of these pairs of clock timestamps therefore provides ameans to synchronize these two devices base on these pair observations.Although there is jitter due to device interrupts used to provide thetimestamps, and delays due to times of RF transmission and codeexecution, their effect may be compensated for. For example, a constantdelay may be readily accounted for. Similarly, jitter may be compensatedfor by statistical methods. An example of messaging system in which suchtimestamps may be observed is the Snobee Data Request broadcastmessaging protocol.

The observed timestamp pairings eventually reach the server that iscoupled to the wireless mesh network. In an example, a timestamp pairing(D1, D2) results when a message is broadcast by device D1 and receivedby D2. Device D3 is also within the broadcast range of device D1, andtherefore receipt by device D3 of the same message broadcast by deviceD1 results in another timestamp pairing (D1, D3). In a further messagethat is broadcast from D2 to D3, a further timestamp pairing may beobserved (D2, D3). By virtue of propagation of these messages to theserver, the server is in a position to receive numerous observations oftimestamp pairings of various pairs of neighboring nodes thatcommunicate with one another. In particular, over a time period, theserver will receive multiple observations of the same (D1, D2) timestamppairings. These multiple observations may then be analyzed by the serverto identify the relationship of the respective timers in devices D1 andD2. In an exemplary analysis, the relationship of the respective timersin devices D1 and D2 may be represented by a linear fit, where the slopeof the linear fit represents the relative drift of the respective timersin devices D1 and D2. Similarly, the constant in the linear fitrelationship represents the offset in the respective timers in devicesD1 and D2.

The linear fit process is typically an iterative process, with thenumber of iterations possibly being a configurable number. With anyiterative process, failure conditions are often identified such that areset or other process is initiated when the linear fit process fails.For example, one or more of the following situations may result in afailure determination: (1) no solution is determined within theconfigurable number of iterations (e.g., 10 iterations), (2) the numberof remaining valid points in the linear fit buffer is below aconfigurable number of valid points (e.g., 5), or (3) the averageresidual per valid remaining data point exceeds a configurablethreshold. A failure would result in a determination that a device clockpair has not been “locked”, i.e., the device clock pair is unlocked. Apair may be unlocked when there are an insufficient number of datapoints or the quality of the linear fit is poor. In such an unlockedscenario, the server is notified when a device or gateway resets andimmediately unlocks the tracking pairs involving the device or gatewayand clears the associated fit buffers.

Each device pair may receive a determination of its status, eitherunlocked or locked. To facilitate management of the device clockpairings, the server may maintain the observed device pairings in astructure that captures the relevant data for each unique pairings.Because for purposes of analysis and tracking, the pairings (D1, D2) and(D2, D1) are the same, the same structure may be used for both pairings.

Acoustic (Ultrasound) Schedule

The purpose of time synchronization in a real-time location system is toensure that, for example, each location transmitter in the real-timelocation system is provided a schedule for transmitting an acoustic(e.g., ultrasonic) message such that all location transmitters (at leastwithin a cluster) transmit at the same time. For example, for a scheduleof transmitting one acoustic message per second, it is desirable thateach location transmitter (i.e., node) in the mesh network transmits itsparticular acoustic message at the same time as the other locationtransmitters. By ensuring that all location transmitters transmit at thesame time, the design of the location tags (that receive the acousticmessage) is greatly simplified. Furthermore, the battery life of thelocation tags is greatly extended by such a synchronized (simultaneous)acoustic message broadcast. To ensure such synchronization, each devicerequires an individualized schedule to be provided to it, where suchschedule uses values in terms of the local clock values of that device.Following the completion of locks of the various device pairs in thenetwork and the selection of a reference clock in the network, theserver provides such an individualized schedule to the various locationtransmitters in the mesh network.

After a sufficient number of pair locks have occurred, the server mayselect a reference clock. The acoustic (ultrasonic) schedule for eachdevice is set by the reference clock, and an acoustic schedule iscomputed based on the local clock for the particular device. Theacoustic schedule contains an offset and a period with respect to thelocal device node. The reference clock sets the period of the acousticschedule for all location transmitters. A reference clock may beselected based on such factors as being in the physical middle of themesh network such that the path lengths from the reference clock to thelocation transmitters are minimized. If the reference clock becomesunavailable, the server may pick another reference clock in the meshnetwork. One approach to the selection of another reference clock is toattempt to preserve the offset while changing to the slightly differentperiod of the new reference clock. Another approach to the selection ofanother reference clock is to use a synthesized clock located at theserver to which all devices are synchronized.

The server continues to monitor the observation clock pairs, and updatesthe fit between the observation clock pairs. An updated schedule for adevice node is forwarded to the device node as required. In order tosave resources, an update may be limited to one per five minutes inorder to save resources. The five minutes is merely exemplary, and isconfigurable. Although much longer durations between updates may bepossible, shorter durations between updates may be required in order topromptly deal with the loss of the reference clock.

Propagation of acoustic schedules to their respective device nodes maybe accomplished by identification of an optimal routing through the meshnetwork to the location transmitter. Routing often requires a graphicalunderstanding of the connection paths available between the variousnodes in the mesh network. Optimal routing may be based on the strengthof the wireless signal strength along various possible connections pathsto the location transmitter. Based on the determined pathway between thereference clock and the clock of the location transmitter of interest,the acoustic schedule may be determined by a linear combination of eachdevice pair along the determined pathway to form the desired mappingbetween the reference clock and the location transmitter clock.

FIG. 1 is a perspective representation of location determination system100. Location determination system 100 can be a real-time locationsystem in an environment that determines the location of a moveableobject or person. Placed within environment 102, location determinationsystem 100 can include one or more acoustic transmitting devices 104,mesh network 106, remote processing server 108, modulated acousticsignals 110 and one or more location tags 112.

These components cooperate to provide a location determination system100 the capability of estimating a location of each location tag 112within environment 102. In certain embodiments, location information maybe three-dimensional location information. In typical embodiments,location determination system 100 includes more than one instance ofacoustic transmitting device 104 installed throughout a building orseries of rooms and more than one instance of location tag 112 attachedto, or incorporated into/onto, people, machines, animals, vehicles,robots, stock, equipment, or other objects. Environment 102 can consistof rooms in a building such as, for example, a ward in a hospital, anoffice in an office building, or a storage space in a warehouse. Morethan one instance of environment 102 can exist and include more than oneinstance of acoustic transmitting device 104 and mesh network 106. Whenseveral instances of environment 102 serve one location, building, orcomplex, these instances of environment 102 can be incorporated intoclusters, groups, or management entities. In an alternate embodiment,environment 102 comprises a single room.

Acoustic transmitting device 104 includes an acoustical device, such asan ultrasonic sounder, and processing logic to transmit modulatedacoustic signals 110. Modulated acoustic signals 110 transmitted byacoustic transmitting device 104 communicate an identifier that isunique to a specific instance of acoustic transmitting device 104. In anembodiment, acoustic transmitting device 104 modulates/encodes theidentifier on an ultrasonic carrier having an ultrasonic frequency suchas, for example, about 20 kHz, 40 kHz, or any other suitable ultrasonicfrequencies. As described above, location determination system 100 caninclude more than one instance of acoustic transmitting device 104, andeach acoustic transmitting device 104 can be configured to transmitmodulated acoustic signals 110 containing an identifier unique to thatinstance of acoustic transmitting device 104. Also, in some embodiments,location determination system 100 comprises more than one instance ofacoustic transmitting device 104 disbursed throughout multiple instancesof environment 102.

Location tags 112 include a microphone capable of receiving modulatedacoustic signals 110 from acoustic transmitting device 104, and mayinclude as well as a processing unit to sample, decode, detect andprocess any received modulated acoustic signals 110. Location tag 112resides inside environment 102, and is typically not placed immediatelyadjacent to the walls or the ceiling. Location tags 112 may be aportable device, and may be attached to a person or item of equipment.In some embodiments, location tags 112 are devices such as, for example,a cell phone, an acoustic transducer, an ultrasound transducer, anacoustic tag, an ultrasound tag, and/or any other suitable devices.

In some embodiments, location tags 112 do not perform processing usingtheir own processing units (or do not have their own processing units),but offload the processing to a remote computer such as remoteprocessing server 108 by transmitting relevant data to remote processingserver 108 using one or more appropriate communication channels, e.g.,acoustic, ultrasound, or radio frequencies. Location tags 112 and/oracoustic transmitting device 104 include a wired or wirelesstransmitter, such as a radio transmitter, for transmitting informationrelevant to a real-time location determination. In some embodiments,location tags 112 communicate with remote processing server 108 viaradio frequencies, a Local-Area Network (LAN), a Wide-Area Network(WAN), or other communication network or protocol.

Mesh network 106 self-organizes its members to provide communicationpathways through the network from the end nodes to central server 108,or to a gateway preceding central server 108. Members of mesh network106 include two or more acoustic transmitting devices 104. End nodesinclude an acoustic transmitting device 104 and/or a location tag 112.Communication pathways support messaging and/or signaling carrying timeclock information to support time synchronization of real-time locationdevices to a reference clock. Communications propagate through meshnetwork 106 containing timing information between members of meshnetwork 106. In an embodiment, the timing information includestime-stamp pairs, wherein each time-stamp pair is an indication of therelative clock information of each of the two devices involved in thecommunication. A message or other signal captures this time-stamp pairinformation for the two devices involved, and forwards this informationto central server 108 via mesh network 106. By capturing time-stamp pairinformation for each of two neighboring devices in the mesh network, andforwarding same to central server 108, the relative time pairings forall members of the mesh network may be determined. Based on theserelative time pairings (i.e., offsets) and the use of a synthesizedreference clock in central server 108, time synchronization of allmembers of the mesh network may be achieved. Time-stamp pairs for thesame devices may be determined over time and forwarded to central server108. These time-stamp pairs form a statistically meaning sample of datafrom which to determine relative clock offset and drifts over time forthe members of mesh network 106.

Remote processing server 108 (e.g., central server) consists of one ormore servers processing real-time location data constituting, forexample, identities of location tags and acoustic transmitting devices,locations of acoustic transmitting devices, RF access points, etc.Remote processing server 108 employs standard communication modules(e.g., RF, wireline) to listen for, process, and respond to incomingsignals. Remote processing server 108 includes processes to performoperations and calculations and radio frequency modules to transmitsignals back to mesh network 106 and acoustic transmitting device 104.In an alternate embodiment, remote processing server 108 communicates toacoustic transmitting device 104 and mesh network 106 via a LAN, WAN, orother wireless/wired communication network.

Remote processing server 108 includes a database, which storesinformation about acoustic transmitting devices 104 and location tags112 and tracks locations in real-time. In an embodiment, the databasemay be any commercially available database management system such asMicrosoft Access, Microsoft SQL server, an Oracle database, an IBMdatabase, etc. The database maintains communicative connections to theprocessing elements via traditional networking infrastructure such asrouters, switches, hubs, firewalls, etc. In an embodiment, the databasemay be situated in one computer workstation. Remote processing server108 implements a centralized storage area network, network-attachedstorage, redundant array of independent disks, and/or any otherconfiguration of storage devices to supply sufficient storage capacityto archive the full panoply of locational information. Sufficientstorage alternatively exists in any other physically attached magneticstorage, cloud storage, or any additional storage medium. In anembodiment, remote processing server 108 deploys a commonly utilizedhard-disk interface, such as ATA, SATA, SCSI, SAS, and/or fiber forinterfacing with a storage medium.

In an embodiment, remote processing server 108 receives the multitude oftime pairings from wireless mesh network 106. Remote processing server108 performs calculations and employs statistical models to determine arelative clock offset and drift from a reference clock for all membersof mesh network 106. Remote processing server 108 then transmits theclock offset information back through mesh network 106 to each member.In another embodiment, remote processing server 108 utilizes existingwireless or wired infrastructure to return the clock characteristics orderived schedules to all members of a location determination system 100.

Modulated acoustic signals 110 include an assemblage of signalstransmitted from acoustic transmitting devices 104 that propagate withinenvironment 102. In an embodiment, modulated acoustic signals 110 fallin the ultrasonic range, i.e., 20 kHz up to 10 MHz and beyond. Specificembodiments include modulated acoustic signals at 20 kHz, and at 40 kHz.Location determination system 100 modulates, encodes, identifies, anddetects/decodes modulated acoustic signals 110 in order to differentiateamong various signals and determine locations.

Modulated acoustic signals 110 can include data descriptive ofcharacteristics of the acoustic signals including, for example, a soundpressure level, signal coding type, signal identification, signaldirection normal, signal spatial distribution, signal period, and/orother suitable data. Modulated acoustic signals 110 can further includedata associated with an environment covered by location determinationsystem 100. Such environmental data can include a layout ororganizational hierarchy of the environment, identifying data of thelocation within the environment (e.g. room, area, space, region,building, etc.) in which acoustic transmitting device 104 sits,dimensional specifications of one or more reflective surfaces (e.g.walls, ceilings, floors, objects, etc.) within the environment (e.g.within a room, area, region, etc. in which acoustic transmitting device104 is located), data indicative of the relative location of acoustictransmitting device 104 within the environment, such as an identifier ofthe surface on which acoustic transmitting device 104 is located and/ora location and/or orientation of acoustic transmitting device 104 withrespect to the surface. The environmental data can further includeatmospheric data indicative of the speed of sound, temperature,pressure, humidity, etc. within the environment. In certain embodiments,because the environmental data changes over time, the environmental datamay be frequently updated to reflect current environmental conditions.

Modulated acoustic signals 110 include a code identifier. In anembodiment, mesh network 106 receives modulated acoustic signals 110 andutilizes the received code identifier to determine the identity of theinstance of acoustic transmitting device 104 that transmitted thosemodulated acoustic signals 110. In an alternate embodiment, mesh network106 forwards the sequence of codes to remote processing server 108 via awireless network connection, a Local-Area Network (LAN), a Wide-AreaNetwork (WAN), or other communication network or protocol. In thisembodiment, remote processing server 108 performs the processing anddetermines the identifier of acoustic transmitting device 104.

Coordinating Time Via a Mesh Network

Time synchronization between network devices may occur utilizing astandard star/tree network topology. However, as noted above, such anapproach mandates expensive additional infrastructure requirements forthe intermediate gateways, e.g., expensive power drops, hubs, androuters that must be installed in order to facilitate communicationpathways when operating in a standard star/tree network topology.

In embodiments of the present disclosure, a mesh network infrastructuresystem is described that circumvents these additional infrastructurerequirements. Utilizing a self-organizing and self-defining mesh networkallows acoustic transmitting device 104 and one or more location tags tosynchronize to a reference clock. By self-organizing into a mesh networkand passing time pairings between mesh members to a central server, thesystem can statistically determine a time offset and time drift for eachparticular member in the mesh network. Then, utilizing these devicespecific timing characteristics, the overall system coordinatestransmissions to a particular sub-interval in order to conserve powerresources.

FIG. 2 depicts mesh network 200, according to embodiments of the presentdisclosure. Mesh network 200, depicted in FIG. 1 as mesh network 106,consists of mesh members 202, described in FIG. 2 as mesh members 202-A,mesh members 202-B, mesh members 202-C, etc. A mesh member may be anacoustic transmitting device 104 or a location tag 112. Communicationsfrom one mesh network member to any other mesh network member within thephysical wireless communication footprint from the first mesh networkmember may be achieved with the mesh network approach. Neighboring nodesin mesh network 200 agree on a wake-up rendezvous time window duringwhich messages/signals are exchanged. The exchanged messages/signalsinclude timing information, such as time-stamp pair that is anindication of the relative clock information of each of the two devicesinvolved in the particular communication. Embodiments may exchangemessages/signals that include timing information on a relatively slowschedule, e.g., one per minute.

Mesh network 200 may use any physical layer approach suitable fornetwork communication. For example, mesh network 200 may be implementedusing a low power radio mesh network (e.g., Zigbee) that has a physicalfootprint of 30 to 50 meters when operating in a radio frequency rangefrom 2.4 GHz to 2.5 GHz. Other alternative frequencies include otherindustrial, scientific and medical (ISM) bands such as the 433 MHz, 868MHz and 915 MHz bands. The Zigbee mesh network is a non-limiting exampleof a mesh network. Furthermore, embodiments of the approach describeherein do not place a limit on the number of “hops” that amessage/signal makes in traversing through mesh network 200. However,there is a message loss that increases with the number of “hops” fromthat a message/signal encounters in propagating through mesh network200. Mesh network 200 may not only provide a self-organizing meshnetwork, but may in an embodiment also determine a minimum path for amessage or signal to traverse from one network node to another networknode in mesh network 200. For real-time location system 100, meshnetwork 200 does not need to meet any strict transport layerrequirements. For example, the amount of latency incurred when amessage/signal propagates through mesh network 200 is not a criticalfactor for use of mesh network 200 in real-time location system 100.

FIG. 3 illustrates a flowchart diagram of a method of synchronizing theclocks of one or more mesh node members 202 (i.e., acoustic transmittingdevices 104 and location tags 112) with a reference clock utilizing meshnetwork communications, according to embodiments of the presentdisclosure. Mesh synchronization method 300 consists of receive observedclock pairs step 301, observed clock pair analysis step 302, selectreference clock step 303, and propagate acoustic schedule step 304.

In receive observed clock pairs step 301, central server 108 receives anumber of observed clock pairs from mesh network 106. The mesh networkis self-organizing and self-configuring, and each member of mesh network106 aims to connect to receive and transmit communications from and toall other members of mesh network 106 that fall within a transmissionrange. In an embodiment, these communications occur through a standardwireless protocol (e.g., Bluetooth, WLAN, Zigbee, etc.). A clock pairingresults when a message is broadcast wirelessly from one node in the meshnetwork and received by a second node in the mesh network. When one nodebroadcasts a message, such a message may include a timestamp from theclock of the originating node. Upon receipt of the message, a timestampfrom the clock of receiving node may be added to the message.Consequently, with each received message, a pair of clock timestamps isobserved, one timestamp from the broadcasting node and one timestampfrom the receiving node. Thus, a pairing is a pair of two timestamps,with one timestamp provided by each clock of the two respective devicesinvolved in the original communication. For example, mesh member 202-Amay communicate with mesh member 202-B, whereby a pair is formed byassociating timestamp from mesh member 202-A and timestamp from meshmember 202-B from mesh member 202-B.

In observed clock pair analysis step 302, mesh synchronization method300 determines a best fit for a time pairing between each of the twodevices that resulted in the origination of the timing message thattraversed mesh network 106. As noted above, a pairing is a pair of twotimestamps, with one timestamp provided by each clock of the tworespective devices involved in the original communication. For example,mesh member 202-A may communicate with mesh member 202-B, whereby a pairis formed by associating timestamp from mesh member 202-A and timestampfrom mesh member 202-B. For example, over a time period, the centralserver 108 will receive multiple observations of the same timestamppairings. These multiple observations may then be analyzed by the serverto identify the relationship of the respective timers in the respectivemesh members. In an exemplary analysis, the relationship of therespective timers in respective mesh members may be represented by alinear fit, where the slope of the linear fit represents the relativedrift of the respective timers in the mesh members. Similarly, theconstant in the linear fit relationship represents the offset in therespective timers in the mesh members.

In select reference clock step 303, mesh synchronization method 300examines the various mesh members to select a reference clock. In oneapproach, the reference clock may be selected based on its location inthe middle of the environment 102 in order to reduce the path lengthsfrom the reference clock to the location transmitters.

In propagate acoustic schedule step 304, the transmit schedule ispropagated to mesh member 202-A. In an approach, the transmittedacoustic (ultrasound) schedule includes the determined time offset anddrift to mesh member 202-A so that its acoustic transmissions aresynchronized with all other devices in real-time location system 100.For example, a time offset of 24 is communicated to mesh member 202-A.Upon receipt of this communication, mesh member 202-A knows that itsclock is 24 clock cycles ahead of the reference clock, and adjusts itstime windows for transmission and reception accordingly.

FIG. 4 depicts an example real-time location system 400 that can be usedto implement the methods and systems of the present disclosure. In someimplementations, real-time location system 400 is a real-time locationsystem configured to determine the locations of persons and objects.Real-time location system 400 can be implemented using a client-serverarchitecture that includes mesh network 106 that communicates with oneor more remote computing devices, such as remote processing server 108.Real-time location system 400 can be implemented using other suitablearchitectures.

As shown, real-time location system 400 can include a mesh network 106,an acoustic transmitting device 104, and a location tag 112. In anembodiment, location tag 112 is any suitable type of mobile computingdevice, such as a smartphone, tablet, cellular telephone, wearablecomputing device, or any other suitable mobile computing device. In someimplementations, location tag 112 is a dedicated tag (e.g. passive oractive) or other device for use in the real-time location system.Location tag 112 can include one or more processors 402 and one or morememory devices 404.

One or more processors 402 can include any suitable processing device,such as a microprocessor, microcontroller, integrated circuit, logicdevice, one or more central processing units (CPUs), graphics processingunits (GPUs) dedicated to efficiently rendering images or performingother specialized calculations, and/or other processing devices, such asa system on a chip (SoC) or a SoC with an integrated RF transceiver. Oneor more memory devices 404 can include one or more computer-readablemedia, including, but not limited to, non-transitory computer-readablemedia, RAM, ROM, hard drives, flash memory, or other memory devices.

One or more memory devices 404 can store information accessible by oneor more processors 402, including instructions 406 executed by one ormore processors 402. For instance, one or more memory devices 404 canstore the instructions 406 for implementing one or more modulesconfigured to implement acoustic transmitting device 104, mesh networkmember 202-A, remote processing server 108, and/or other suitableinstructions.

Each of acoustic transmitting device 104, mesh member 202-A, locationtag 112 and remote processing server 108 can include computer logicutilized to provide desired functionality. Thus, each of acoustictransmitting device 104, location tag 202-A, remote processing server108 can be implemented in hardware, application specific circuits,firmware and/or software controlling a general purpose processor. In oneembodiment, each of acoustic transmitting device 104, mesh member 202-A,location tag 112 and remote processing server 108 are program code filesstored on a storage device, loaded into memory and executed by aprocessor, or provided from computer program products, for examplecomputer executable instructions stored in a tangible computer-readablestorage medium such as RAM, hard disk or optical or magnetic media.Acoustic transmitting device 104, mesh member 202-A, location tag 112and remote processing server 108 can each correspond to one or moredifferent programs, files, circuits, or sets of instructions. Likewise,two or more instances of acoustic transmitting device 104, mesh member202-A, location tag 112 and remote processing server 108 can be combinedinto a single program, file, circuit, or set of instructions.

Instructions 406 can further include instructions for implementing abrowser, for running a specialized application, or for performing otherfunctions on location tag 112. For instance, the specialized applicationcan be used to exchange data with remote processing server 108 over thenetwork 420. Instructions 406 can include client-device-readable codefor providing and implementing aspects of the present disclosure. Forexample, instructions 406 can include instructions for implementing anapplication associated with location determination system 100 or a thirdparty application implementing asset tracking or other services onlocation tag 112.

Location tag 112 can also include data 408 that one or more processors402 retrieves, manipulates, creates, or stores. Data 408 can include,for instance, identifiers, sequences of codes, random numbers, acousticmodel data, sensor data, and/or other data. Location tag 112 can includevarious input/output devices for providing and receiving informationfrom a user, such as a touch screen, touch pad, data entry keys,speakers, and/or a microphone suitable for voice recognition. Forinstance, location tag 112 can multiple input buttons signifyingdifferent events. In an exemplary embodiment, a person in a hospitalcould press a button to signal distress.

Location tag 112 can further include receiver 410. Receiver 410 can beany device or circuitry for receiving, listening for, decoding,interpreting or otherwise processing modulated acoustic signals 110 fromacoustic transmitting device 104. Location tag 112 can also include anetwork interface used to communicate with remote processing server 108or acoustic transmitting device 104 over network 420. The networkinterface can include any suitable components for interfacing with onemore networks, including for example, transmitters, receivers, ports,controllers, antennas, or other suitable components. Location tag 112can further include a communication system used to communicate withacoustic transmitting device 108. The communication system can include,for instance, one or more transducers (e.g. microphone devices)configured to receive acoustic (e.g. ultrasonic) signals from acoustictransmitting device 104.

In some implementations, location tag 112 can be in communication with aremote computing device, such as remote processing server 108 overnetwork 420. Remote processing server 108 can include one or morecomputing devices. Remote processing server 108 can include one or morecomputing devices and can be implemented, for instance, as a parallel ordistributed computing system. In particular, multiple computing devicescan act together as a single remote processing server 108.

Similar to location tag 112, remote processing server 108 can includeone or more processors 412 and memory 414. One or more processors 412can include one or more central processing units (CPUs) and/or otherprocessing devices. Memory 414 can include one or more computer-readablemedia and store information accessible by one or more processors 412,including instructions 416 that can be executed by one or moreprocessors 412 and data 418.

Data 418 can be stored in one or more databases. The data can includeidentifier information, acoustic model data, and other data required bylocation determination system 100. One or more databases can beconnected to remote processing server 108 by a high bandwidth LAN or WANor can also be connected to remote processing server 108 through network420. The one or more databases can be split up and reside in distributedor multiple locales.

Remote processing server 108 can also include a network interface usedto communicate with acoustic transmitting device 104, mesh member 202-A,and location tag 112 over network 420. The network interface can includeany suitable components for interfacing with one more networks,including for example, transmitters, receivers, ports, controllers,antennas, or other suitable components.

Network 420 can be any type of communications network, such as a localarea network (e.g. intranet), wide area network (e.g. Internet),cellular network, or some combination thereof. Network 420 can alsoinclude direct connections between acoustic transmitting device 104,mesh member 202-A, location tag 112, and remote processing server 108.Network 420 can include any number of wired or wireless links andinstrumented using any suitable communication protocol.

Location system 400 can further include one or more instances ofacoustic transmitting device 104. Acoustic transmitting device 104 cantransmit acoustic signals (e.g. ultrasonic signals) as described inFIG. 1. In some implementations, acoustic transmitting device 104 cantransmit other suitable signals, such as radio frequency signals.Acoustic transmitting device 104 can be implemented using any suitablecomputing devices. Acoustic transmitting device 104 can include one ormore transducers configured to emit acoustic or other suitable signalsthat location tag 112 uses to derive a location. Although FIG. 4 depictsonly one acoustic transmitting device 104 and location tag 112 it willbe appreciated by those skilled in the art that any suitable number ofthese devices can be included in location system 400.

It is to be appreciated that the Detailed Description section, and notthe Summary and Abstract sections, should be used to interpret theclaims. The Summary and Abstract sections may set forth one or more butnot all exemplary embodiments of the present invention, and thus, shouldnot limit the present invention and the appended claims in any way.

The invention has been described above with the aid of functionalbuilding blocks illustrating the implementation of specified functionsand relationships thereof. The boundaries of these functional buildingblocks have been arbitrarily defined herein for the convenience of thedescription. Alternate boundaries may be defined so long as thespecified functions and relationships thereof are appropriatelyperformed.

It will be apparent to those skilled in the relevant art(s) that variouschanges in form and detail can be made therein without departing fromthe spirit and scope of the disclosure. Thus, the invention should notbe limited by any of the above-described exemplary embodiments, butshould be defined only in accordance with the following claims and theirequivalents.

What is claimed is:
 1. A real-time location system in an environment,comprising: a location tag having a location ID, wherein the locationtag is configured to transmit the location ID and a received acoustic IDfrom an acoustic transmitting device to a central server via a wirelessmesh network; the wireless mesh network comprising a first mesh networkmember and a second mesh network member, the first mesh network memberand the second mesh network member being battery-powered devices, thefirst mesh network member having a first clock and the second meshnetwork member having a second clock, wherein the first mesh networkmember transmits a first timestamp of the first clock to the second meshnetwork member, and the second mesh network member generates a messagefor propagation to the central server, the message includingidentification of the first mesh network member and the second meshnetwork member, and the first timestamp and a second timestamp of thesecond clock; a central server configured to select a reference clockwithin the wireless mesh network, further configured to determine a timeoffset between the first clock and the reference clock based on themessage; and further configured to propagate an acoustic transmissionschedule to the first mesh network member, wherein the first meshnetwork member and the location tag communicate acoustically based onthe acoustic transmission schedule.
 2. The real-time location system ofclaim 1, wherein the time offset is determined using statisticalanalysis.
 3. The real-time location system of claim 1, wherein the timeoffset is determined using linear regression analysis with outlierrejection.
 4. The real-time location system of claim 1, wherein thecentral server is further configured to update the acoustic transmissionschedule.
 5. The real-time location system of claim 1, wherein thecentral server is further configured to select a replacement referenceclock.
 6. The real-time location system of claim 1, wherein the wirelessmesh network uses a Zigbee protocol.
 7. The real-time location system ofclaim 1, wherein the wireless mesh network communicates within a radiofrequency range of 2.4 to 2.5 GHz, or within an 433 MHz, 868 MHz or 915MHz ISM band.
 8. The real-time location system of claim 1, wherein thelocation tag receives a firmware upgrade via the wireless mesh network.9. The real-time location system of claim 1, wherein the wireless meshnetwork facilitates propagation of the acoustic transmission schedulevia intermediate connections based on wireless signal strength.
 10. Thereal-time location system of claim 1, wherein the reference clock is apart of the central server.
 11. A method for utilizing a wireless meshnetwork, comprising: receiving, by a location tag, an acoustic ID froman acoustic transmitting device; transmitting, by the location taghaving a location ID, the location ID and the received acoustic ID to acentral server via a wireless mesh network, the wireless mesh networkcomprising a first mesh network member and a second mesh network member,the first mesh network member and the second mesh network member beingbattery-powered devices, the first mesh network member having a firstclock and the second mesh network member having a second clock;transmitting by the first mesh network member a first timestamp of thefirst clock to the second mesh network member; generating a message bythe second mesh network member for propagation to the central server,the message including identification of the first mesh network memberand the second mesh network member, and the first timestamp and a secondtimestamp of the second clock; determining by the central server areference clock within the wireless mesh network, and determining a timeoffset between the first clock and a reference clock based on themessage; and propagating an acoustic transmission schedule to the firstmesh network member, wherein the first mesh network member and thelocation tag communicate acoustically based on the acoustic transmissionschedule.
 12. The method of claim 11, wherein the determining usesstatistical analysis.
 13. The method of claim 11, wherein thedetermining uses linear regression analysis with outlier rejection. 14.The method of claim 11, further comprising: updating the acoustictransmission schedule.
 15. The method of claim 11, further comprising:selecting a replacement reference clock.
 16. The method of claim 11,wherein the wireless mesh network uses a Zigbee protocol.
 17. The methodof claim 11, wherein the wireless mesh network communicates within aradio frequency range of 2.4 to 2.5 GHz, or within an 433 MHz, 868 MHzor 915 MHz ISM band.
 18. The method of claim 11, further comprises:receiving, by the location tag, a firmware upgrade via the wireless meshnetwork.
 19. The method of claim 11, wherein the propagating an acoustictransmission schedule to the first mesh network member includes:propagating the acoustic transmission schedule via intermediateconnections based on wireless signal strength.
 20. The method of claim11, wherein the reference clock is a part of the central server.